In theory, WaniKani chose the SRS interval levels because assuming nothing gets in the way, performing the reviews immediately as they come up is the around the proper range of time to work with how your brain utilizes information. Of course, everyone learns at different speeds, and most people have lives that make absorbing Japanese constantly more difficult than in theory.
Take too long to review and you’ll forget things. Go too fast and you’ll mess up a lot and/or burn out. Finding that sweet spot takes knowing yourself well. A lot of people have reset their level quite a way back because they realized their pace (either too fast or too slow) was affecting their performance. The smartest thing I’ve seen many people do is focus on speed-running the radicals and kanji only while taking the vocabulary at whatever pace they need. That way you can access more content quickly yet not overwhelm yourself.
If you consider how many functions of modern society works, it makes sense that there’s an impulse to do something when you can. “Available Now” just strikes a weird cord that makes people think, “I have to do it now!” That’s especially seen in the numerous posts from level 1-3s who constantly remark that WaniKani is too slow (and hence why it’s a combination of a community joke and pet peeve).
Edit: @chromattic Also, to contribute to your post, I have 2,537 active items (3,695 including burned) with 5 leeches (4 legitimate, 1 typo).